simulate a differential ring oscillator 4stage on ads2008

i have a problem for simulate a differential ring oscillator 4stage on ads2008. how to insert a intial condition for input differential

A small piece-wise linear current source can be used as a trigger.It will act as a starter at the beginning of the simulation then stops.
That's what I'm using in oscillator simulations.Voltage source can also be used.

hi , thanks BigBoss but how i do this work and i use which voltage source or current.
i have a other question , how i access to recently work about a subject?

Add a PW Current Source at anywhere on the loop and excite the oscillator by adding a small current for a while. Injection of a small current at the beginning of the simulation will excite the simulator and the oscillator will start to swing.
You have to try to find the best point ( because any node may not be proper ).